Buck-boost converters may be used as drivers for loads with specific current and/or voltage requirements. A buck-boost converter may supply an output voltage that is greater than, less than, or equal to the input voltage. A non-inverting buck-boost converter may be used to supply an output voltage with the same polarity as the input voltage. A non-inverting buck-boost converter may use four switches connected around a single common inductor, with the switches controlling when the buck-boost converter operates in buck mode (i.e., output voltage lower than input voltage) or boost mode (i.e., output voltage higher than input voltage).
A non-inverting buck-boost converter may be controlled to operate in pulse frequency modulation (PFM) mode, typically when the required load current is relatively low. The converter may accumulate current energy in the inductor that the converter may then discharge in pulses or packets of current energy from the inductor to the output of the converter. The converter may discharge a train or series of multiple packets of energy (also referred to as bursts) to the output of the converter beginning when the output voltage falls below a triggering threshold and ending once the output voltage has risen to a sufficiency threshold. The converter may have an output capacitor coupled to the output that stores charge at the output.
The converter may then remain inactive while the output voltage lowers as the load on the output draws charge from the output capacitor. The output voltage may lower past the triggering threshold, and prompt the converter to begin discharging a train of inductor energy packets again. The converter therefore alternates its output voltage around two thresholds, the lower, triggering threshold and the higher, sufficiency threshold. The converter may experience a ripple voltage and overshoots beyond the voltage thresholds. Any converter also generates some energy loss between input and output. A converter's efficiency may be defined as a ratio of its output energy to its input energy. For a non-inverting buck-boost converter being operated in PFM mode, energy losses are typically due more to switching and dynamic losses (due to switch gate capacitance and V*I power dissipated across the switches during transitions) than to ohmic losses.